S5,S15,S10成等差数列,
2S15=S5+S10
2a1(1-q^15)/(1-q)=a1(1-q^5)/(1-q)+a1(1-q^10)/(1-q)
2-2q^15=1-q^5+1-q^10
2q^15=q^5+q^10
除以q^4
2q^11=q+q^6
2a1q^11=a1q+a1q^6
2a12=a2+a7
所以a2,a12,a7成等差数列